Old Bill is spot on (5)
I believe the answer is:
exact
'spot on' is the definition.
(I've seen this before)
'old bill' is the wordplay.
'old' becomes 'ex'.
'bill' becomes 'act' (I've seen this in other clues).
'ex'+'act'='EXACT'
'is' is the link.
(Other definitions for exact that I've seen before include "Precise, accurate" , "totally accurate" , "Spot-on" , "Demand and obtain" , "Finished" .)